fix #21305, inference of splatting T.types or T.parameters

[JeffBezanson]

The first commit here is uncontroversial; it simply allows optimizing f(x...) when x is a Const tuple or SimpleVector. Previously, this would not get optimized when x was a Const(svec(...)).

The second commit infers a Const for T.parameters when T's type is Type{T}, and T is a leaf type. We were rightly being very cautious here, but I think this should be safe for leaf types, since they're all normalized via the type cache to have a predictable structure.